Lots of guitar amps run the 6V6 well in excess of its ratings, and they get away with it. Solid state though... it seems now that datasheet ratings are pretty optimistic for some transistors.

Manufacturers will do everything to milk extra watts from their products so that they can advertise the higher values.
This is not new, it's been going on since the 1950's.
This also comes at the expense of the life of said equipment.
In the case of tubes, it gave the servicers a job, changing tubes, etc...
In the case of solid state, it also influenced the buying of new equipment.
Lower the price, run it hot, when it fails, make a new sale.

Doesn't sound like good engineering to me - perhaps they were forced to substitute a lower voltage part when the original went end-of-line? Or someone ordered 10000 of the wrong chip? It is rare to see an audio chip higher than +/-18V rated though, I am surprized at the 23V rails. Clearly the chip failed due to voltage stress and its not clear how to proceed unless you can source a higher rated part.

Doesn't sound like good engineering to me - perhaps they were forced to substitute a lower voltage part when the original went end-of-line? Or someone ordered 10000 of the wrong chip? It is rare to see an audio chip higher than +/-18V rated though, I am surprized at the 23V rails. Clearly the chip failed due to voltage stress and its not clear how to proceed unless you can source a higher rated part.

I would have changed the regulator zeners (mentioned in a previous post) to 18V long ago, and called it a day.

I have now repair the amplifier but not after some troubles.
First off all why have they chosen to use this IC(LC78212 analog switch), the only function is has, is get commands from the microcontroller over serial to which darlington transistor to turn on that is connected to a relay. To switch between the different inputs.
Why have they not just used a shift register to this, because there is not coming any analog signal through the IC, and 23V is not necessary to turn on the Darlington transistors(only 3V)?
My guess: they are probably using it in a lot of other product, and didn't wanted to use a new part just for this product.

Now for the repair:
I ordered a new IC(analog switch) installed it, and inserted 12V zener diodes so the supply voltages only was +- 12V. But it was still not able to switch between the different inputs. Then I found out there was no signal from the microcontroller to the switch. The microcontroller was still running as I could see the LEDs in the front panel light up for each input. So the IO pins on the was probably burnt off.
As the function of the analog switch was so simple and the front panel LEDs was working, the easy solution was to connect the signal for the LEDs to the Darlington transistor. But the signal just has to be inverted by a simple 74ls04 logic inverter, and that worked!!!